Partial discharges (PD) are locally occurring electric discharges that partially bridge the insulation between conductors, in particular used in high voltage electrical generation.
They are the result of insulation defects when the electrical field is locally beyond the breakdown strength. Such partial discharges may even lead to further degradation of the insulation between the conductor and even breakdown.
Therefore, the monitoring of partial discharges in high voltage systems is needed to prevent dysfunctioning and damages in a power generation chain. In addition, it is a helpful tool in order to plan shut downs and maintenance of the equipment or replacements.
In industry, measurement of partial discharges is well described in standard EN/IEC 60270 entitled “High voltage Test Techniques—Partial Discharge measurements” (2000). It describes design and calibration of electrical circuits used to determine partial discharges.
It is important to notice that before the measurement of partial discharges, the measurement system needs to be calibrated in order to reach the needed accuracy. Calibration is usually made by injecting a pulse into the system and detecting the response of the measurement device.
The classical calibration devices can only be used off-line, at stand still of the generator because the known devices are not high voltage resistant.
However, the operation conditions on-line and off-line are quite different from each other which impacts the calibration precision. In fact, during operation the main switch connecting the generator to the grid is closed and thus the electric line is much longer. This causes a larger line capacitance and changes the transmission characteristics of the pulses.
Thus, it is desirable to be able to calibrate the high voltage insulation monitoring device during normal operation of the generator.
EP 2 071 342 discloses a portable calibration device that might be used during normal operation as the power supply is achieved via a battery and remote controlling is done via an optical interface.
However, it's an add-on device that changes the high voltage geometry of the power excitation chain and due to the battery supply, the injected pulses are quite small and the time of operation limited.
It is an object of the present invention to overcome at least partially the above cited drawbacks, in particular to propose a device that allows on-line calibration, even during high voltage insulation monitoring.